US10954164B2 - Formed hexagonal boron nitride body, heat-treated hexagonal boron nitride body and processes for producing the same - Google Patents

Formed hexagonal boron nitride body, heat-treated hexagonal boron nitride body and processes for producing the same Download PDF

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US10954164B2
US10954164B2 US16/323,454 US201716323454A US10954164B2 US 10954164 B2 US10954164 B2 US 10954164B2 US 201716323454 A US201716323454 A US 201716323454A US 10954164 B2 US10954164 B2 US 10954164B2
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formed body
water
powder
material composition
heat
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US20190177235A1 (en
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Jan C. Zimmer
Ursula Kayser
Krishna B. Uibel
Christoph Lesniak
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3M Innovative Properties Co
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Definitions

  • the present invention relates to a formed hexagonal boron nitride body with a good mechanical stability, to a heat-treated formed hexagonal boron nitride body with a low content of water-soluble boron compounds and good sealing properties, to a material composition for making such a formed hexagonal boron nitride body, and to processes for making such a formed hexagonal boron nitride body and such a heat treated formed hexagonal boron nitride body.
  • Hexagonal boron nitride has a high temperature stability, is electrically insulating, is a solid lubricant and has a high thermal conductivity. Furthermore hexagonal boron nitride is used as raw material for the production of cubic boron nitride, it can be used for cosmetic purposes and as a release agent for metallurgical applications, and for further applications for example in the field of metallurgy. It is densified by hot-pressing in order to obtain sufficient mechanical stability and optionally sufficient density of formed bodies. The hot-pressed formed bodies are usually machined mechanically to the final shape.
  • Hexagonal boron nitride powder is technical synthesized by nitridation of boric acid in the presence of a nitrogen source.
  • a nitrogen source ammonia can be used, and then usually calcium phosphate is used as a carrier material for the boric acid.
  • An organic nitrogen source such as melamine and urea can also be used and can be reacted with boric acid or borates under nitrogen.
  • the nitridation usually is carried out at a temperature of from 800° C. to 1200° C.
  • the thereby obtained boron nitride is substantially amorphous, it is also known as turbostratic boron nitride.
  • the amorphous boron nitride is transformed at higher temperatures of up to about 2100° C., preferably in a nitrogen atmosphere, into hexagonal, crystalline boron nitride. Often for this high temperature treatment crystallization additives are added to the amorphous boron nitride.
  • hexagonal boron nitride In the high temperature treatment, hexagonal boron nitride (hBN) is produced in the form of primary particles having a platelet shaped morphology. Typical sizes of the boron nitride platelets are in the range of 1 to 20 ⁇ m, but platelet sizes of up to 50 ⁇ m are also possible. Usually the hexagonal boron nitride is crushed or de-agglomerated after the high temperature treatment, in order to obtain powders that can be further processed for various applications. In further steps impurities can be removed selectively to obtain high purity hexagonal boron nitride.
  • hexagonal boron nitride powder is filled in molds and hot-pressed and thereby densified at high temperatures.
  • the hexagonal boron nitride powder is granulated before it is hot-pressed, in order to increase the bulk density and to allow higher fillings of the molds, as non-granulated hexagonal boron nitride powders have usually low bulk densities of about 0.1 to 0.5 g/cm 3 .
  • the so produced hot-pressed bodies have a sufficient mechanical stability for producing solid shapes, which is usually done by mechanical machining.
  • Hot-pressing of boron nitride is a cost-intensive process, as for hot-pressing a special equipment is required where high temperatures of above 1800° C. and up to about 2200° C. and high uniaxial pressures of about 20 MPa can be applied in a protective gas atmosphere.
  • EP 0 939 066 A1 discloses a method to form pellets or agglomerates of high density boron nitride made of high-purity hexagonal boron nitride powder, with the operations of cold pressing and granulation repeated until the density of the pellets or agglomerates is above 1.86 g/cm 3 .
  • the obtained pellets are intended to be used for converting hexagonal boron nitride into cubic boron nitride and are mechanically weak.
  • DE 19 750 107 C1 discloses a boron nitride sealing body that is produced by using machined hot-pressed hexagonal boron nitride to which a binder has been added for dry-pressing.
  • a binder aluminum phosphate can be used.
  • DE 10 2014 222 365 A1 discloses a method of manufacturing a seal for a sensor element, comprising the following steps: providing a ceramic material having at least boron nitride and boron trioxide with a proportion of boron trioxide with respect to the ceramic material of 2.0 wt.-% to 6.0 wt.-%, forming the ceramic material by pressing to a gasket, and thermal treatment of the seal at temperatures of 475 to 1250° C.
  • the boron nitride material which is used for the disclosed method can be obtained, for example, by milling or machining of hot-pressed sintered parts.
  • DE 10 2008 062 155 A1 discloses a ceramic mass for producing a sintered hexagonal boron nitride body that is solidifiable in a pressureless thermal treatment, comprises hexagonal boron nitride, and a nano-scale powder based on oxides, hydroxides, oxyhydrates or compounds forming oxides under thermal treatment, of silicon and aluminum and of aluminum.
  • the forming of the ceramic masses is done by extruding with a moisture content of 30 to 34 wt.-%.
  • U.S. Pat. No. 5,320,989 discloses a material and a method of forming a dried cake by mixing boron nitride powder with a source of aluminum oxide, for example a water-based colloidal aluminum oxide or peptized aluminum oxide monohydrate liquid.
  • a source of aluminum oxide for example a water-based colloidal aluminum oxide or peptized aluminum oxide monohydrate liquid.
  • the dried cake can be subsequently pressureless sintered at a temperature of about 500° C. to about 1000° C.
  • the non-published European patent application no. 16174825.6 discloses a formed body comprising hexagonal boron nitride, wherein the formed body has a Brinell hardness of at least 2 HBW 2.5/2, and wherein the formed body is obtainable by a process at temperatures of at most 100° C.
  • the formed body may further comprise water-soluble boron compounds.
  • hexagonal boron nitride formed bodies that are mechanically stable and that can be produced by a cost-efficient process without the need of hot-pressing, and that can be used as sealing components, and that have a reduced content of water-soluble boron compounds.
  • a means that the term should include the singular and plural form.
  • compound(s) means one compound and more compounds (e.g. 2, 3, 4, etc.).
  • the term “comprise” shall include also the terms “consist essentially of” and “consists of”.
  • the subject-matter of the invention is consequently a material composition comprising hexagonal boron nitride, a water-soluble boron compound and a further inorganic compound, wherein the further inorganic compound is able to split off a gaseous phase at a heat treatment at a temperature of at most 1000° C., and wherein the further inorganic compound is able to form with the water-soluble boron compound(s) a water-insoluble boron compound at a heat treatment at a temperature of 200-1000° C.
  • the subject-matter of the invention is furthermore a formed body comprising the material composition according to the invention.
  • the formed body according to the invention comprises hexagonal boron nitride, a water-soluble boron compound and a further inorganic compound, wherein the further inorganic compound is able to split off a gaseous phase at a heat treatment at a temperature of at most 1000° C., and wherein the further inorganic compound is able to form with the water-soluble boron compound(s) a water-insoluble boron compound at a heat treatment at a temperature of 200-1000° C.
  • the subject-matter of the invention is also a heat-treated formed body obtained by heat-treating the formed body according to the invention at a temperature of 200-1000° C., wherein the heat-treated formed body comprises hexagonal boron nitride and the reaction product between the water-soluble boron compound and the further inorganic compound as described above, and wherein the reaction product is a water-insoluble boron compound.
  • the heat-treated formed body according to the invention comprises hexagonal boron nitride and the reaction product between a water-soluble boron compound and a further inorganic compound, wherein the further inorganic compound is able to split off a gaseous phase at a heat treatment at a temperature of at most 1000° C., and wherein the further inorganic compound is able to form with the water-soluble boron compound(s) a water-insoluble boron compound at a heat treatment at a temperature of 200-1000° C., and wherein the reaction product is formed if the reaction partners are heat treated at a temperature of 200-1000° C.
  • the subject-matter of the invention is also a process for making the formed body according to the invention wherein the process comprises the steps
  • step (b) generating at least one water-soluble boron compound by mechanically pre-treating the powder of step (a) and/or by adding at least one water-soluble boron compound to the powder provided in step (a), thereby obtaining a powder comprising hexagonal boron nitride powder and at least one water-soluble boron compound;
  • step (c) adding to the powder of step (b) a further inorganic compound, wherein the further inorganic compound is able to split off a gaseous phase at a heat treatment at a temperature of at most 1000° C., and wherein the further inorganic compound is able to form with the water-soluble boron compound(s) a water-insoluble boron compound at a heat treatment at a temperature of 200-1000° C.;
  • step (d) mixing the powder of step (b) and the further inorganic compound added in step (c), thereby obtaining a material composition comprising hexagonal boron nitride;
  • the subject-matter of the invention is also a process for making the heat-treated formed body according to the invention wherein the process comprises the steps
  • step (b) generating at least one water-soluble boron compound by mechanically pre-treating the powder of step (a) and/or by adding at least one water-soluble boron compound to the powder provided in step (a), thereby obtaining a powder comprising hexagonal boron nitride powder and at least one water-soluble boron compound; (c) adding to the powder of step (b) a further inorganic compound, wherein the further inorganic compound is able to split off a gaseous phase at a heat treatment at a temperature of at most 1000° C., and wherein the further inorganic compound is able to form with the water-soluble boron compound(s) a water-insoluble boron compound at a heat treatment at a temperature of 200-1000° C.
  • step (d) mixing the powder of step (b) and the further inorganic compound added in step (c), thereby obtaining a material composition comprising hexagonal boron nitride; (e) optionally granulating the material composition of step (d), thereby obtaining a material composition comprising hexagonal boron nitride, wherein the material composition is granulated; (f) forming the material composition of step (d) or the material composition of step (e) into a shape, thereby obtaining a formed body; and (g) heat-treating the formed body of step (f) at a temperature of 200-1000° C.
  • the formed body and the heat-treated formed body according to the invention are mechanically stable. They have a high degree of contour accuracy and high edge stability.
  • the formed body and the heat-treated formed body being mechanically stable can be produced without the need of a cost-intensive high-temperature hot-pressing process.
  • the heat-treated formed body according to the invention has a lower content of water-soluble boron compounds than the formed body, as the content of water-soluble boron compounds is reduced by the formation of a water-insoluble boron compound.
  • the heat-treated formed body according to the invention as well as the formed body according to the invention have good sealing capabilities and a low leakage rate for gases.
  • a “water-soluble boron compound” means that at least 4.5 g of boric acid from the water-soluble boron compound can be dissolved in 100 ml water at 23° C. during 1 hour.
  • a “water-insoluble boron compound” means that less than 4.5 g of boric acid from the water-insoluble compound can be dissolved in 100 ml water at 23° C. during 1 hour.
  • the material composition and the formed body according to the invention comprise hexagonal boron nitride, a water-soluble boron compound and a further inorganic compound.
  • the further inorganic compound is able to split off a gaseous phase at a heat treatment at a temperature of at most 1000° C., and the further inorganic compound is able to form with the water-soluble boron compound(s) a water-insoluble boron compound at a heat treatment at a temperature of 200-1000° C.
  • the further inorganic compound splits off a gaseous phase when a heat treatment at a temperature of at most 1000° C. is performed, and the further inorganic compound forms with the water-soluble boron compound(s) a water-insoluble boron compound when a heat treatment at a temperature of 200-1000° C. is performed, but it is not necessary to perform a heat treatment in order to obtain the material composition or the formed body according to the invention.
  • the heat treatment is performed in order to obtain the heat-treated formed body according to the invention.
  • the gaseous phase is water and/or carbon dioxide. More preferably, the gaseous phase is water.
  • the water-soluble boron compound(s) can be oxygen containing boron compounds such as boric acid (H 3 BO 3 ) and ammonium borates.
  • boron compounds such as boric acid (H 3 BO 3 ) and ammonium borates.
  • ammonium borates are the ammonium pentaborate (NH 4 )B 5 O 8 or the hydrate thereof. At least one of these water-soluble boron compounds is present in the material composition and the formed body, it is also possible that a mixture of these water-soluble boron compounds is present in the material composition and the formed body.
  • the material composition and the formed body preferably do not contain boron oxide (B 2 O 3 ) as boron oxide is preferably not added to the material composition, and it is not produced by the process for making the formed body. This is confirmed by X-ray measurements that do not show the diffraction lines of boron oxide.
  • the further inorganic compound can be a metal hydroxide or a metal oxyhydroxide, wherein the metal is preferably selected from the group consisting of aluminum, calcium and magnesium, or combinations thereof. These inorganic compounds split off water at a heat treatment at a temperature of at most 1000° C.
  • the further inorganic compound can also be a carbonate or a hydrogen carbonate of the alkali metals and alkaline earth metals, for example calcium carbonate, calcium hydrogen carbonate, sodium carbonate or sodium hydrogen carbonate, or combinations thereof. These inorganic compounds split off carbon dioxide at a heat treatment at a temperature of at most 1000° C. It is also possible to use as further inorganic compound combinations of the compound(s) that split off water and of the compound(s) that split off carbon dioxide at a heat treatment at a temperature of at most 1000° C.
  • synthetic inorganic compounds are used as further inorganic compound.
  • the high purity and well-defined properties of particle size and particle morphology of the synthetic inorganic compounds guarantee reproducible microstructure and electrical insulation and sealing properties of the formed body and of the heat-treated formed body.
  • the further inorganic compound is a hydroxide or a oxyhydroxide of aluminum, calcium or magnesium.
  • the further inorganic compound can be aluminum hydroxide (Al(OH) 3 ), or boehmite (AlO(OH)). More preferably, boehmite is used as further inorganic compound. Even more preferably, boehmite in the form of nano-scale particles is used as further inorganic compound. The mean particle size of the nano-scale particles is at most 100 nm.
  • the nano-scale particles of boehmite are commercially available as spray-dried products such as Disperal and Disperal P2 from Sasol Germany GmbH and can be dispersed in water by peptization in the presence of an acid.
  • the resulting product is a boehmite sol with mean particle sizes of the dispersed particles below 100 nm.
  • Disperal and Disperal P2 are hydrated boehmites, having physically bound water, additionally to the chemical bound water of the aluminum oxyhydroxide boehmite.
  • hydrated boehmite is used as further inorganic compound, more preferably in the form of nano-scale particles.
  • the mean particle size of the nano-scale particles is at most 100 nm.
  • the material composition and the formed body may contain additional ingredients, besides hexagonal boron nitride, a water-soluble boron compound and the further inorganic compound.
  • Additional ingredients can be, for example, inorganic additives in form of mineral or ceramic powders with a typical particle size (d 50 ) of 0.5 to 10 ⁇ m, depending on the intended use of the formed body.
  • additional ingredients can be oxide ceramics, for example aluminum oxide (Al 2 O 3 ), silicon oxide (SiO 2 ) and magnesium oxide (MgO), carbide ceramics, for example silicon carbide (SiC), and nitride ceramics, for example silicon nitride (Si 3 N 4 ) and aluminum nitride (AlN), and mixtures thereof.
  • the material composition and the formed body have a boron nitride content of at least 15% by weight, preferably at least 35% by weight, more preferably at least 55% by weight, based on the total weight of the material composition or respectively of the formed body.
  • the material composition and the formed body preferably have a boron nitride content of at most 95%, more preferably at most 90%, more preferably at most 85% by weight, based on the total weight of the material composition or respectively of the formed body.
  • the material composition and the formed body have a boron nitride content of at least 55% and at most 85% by weight, based on the total weight of the material composition or respectively of the formed body.
  • the density of the formed body is at least 1.3 g/cm 3 , preferably at least 1.6 g/cm 3 . In some embodiments, the density of the formed body is at least 1.8 g/cm 3 .
  • the relative density of the formed body is preferably at least 50% of the theoretical density of the formed body. More preferably, the relative density of the formed body is at least 60%, more preferably at least 70% of the theoretical density of the formed body. Typically, the relative density of the formed body can be 99% or less, or 95% or less, or 90% or less, of the theoretical density of the formed body.
  • the theoretical density of hexagonal boron nitride formed bodies is calculated by the powder density of the pure hexagonal boron nitride powder which is 2.25 g/cm 3 and the theoretical densities of the other components and their respective fractions in the composition of the formed body.
  • the formed body is mechanically stable and has good mechanical properties. Thus the formed body is stable for transportation.
  • the compressive strength of the formed body preferably is at least 4 N/mm 2 . More preferably, the compressive strength of the formed body is at least 8 N/mm 2 , more preferably at least 10 N/mm 2 . In some embodiments, the compressive strength of the formed body is at least 15 N/mm 2 . In some other embodiments, the compressive strength of the formed body is at least 25 N/mm 2 .
  • the compressive strength of the formed body is measured as maximum force before rupture of a test sample of the formed body, divided by the area of the test sample on which the load is applied.
  • the compressive strength is measured in the pressure test.
  • the pressure test is typically carried out with test samples having the dimensions 20 ⁇ 20 ⁇ 10 mm and being axially pressed with a pressure of 1250 bar.
  • the pressure test can be carried out in a universal testing machine, for example in a universal testing machine type 1474.100 from Zwick GmbH & Co. KG, Germany.
  • the pressure test is carried out at room temperature (23° C.).
  • the pressure test can be carried out according to DIN 51104 with the following differences: the samples for which the pressure test is carried out are in direct contact with the pressure plates of the testing machine, the material of the pressure plates is not a ceramic material but a cemented carbide, and the size of the samples for which the pressure test is carried out is 20 ⁇ 20 ⁇ 10 mm.
  • the hardness of the formed body can be measured by the Brinell method according to DIN EN ISO 6506-1 (2013).
  • the Brinell hardness of the formed body is at least 2 HBW 2.5/2.
  • the designation “HBW 2.5/2” means that the measurement has been carried out with a Brinell sphere of diameter 2.5 mm and with a load of 2 kp.
  • the Brinell hardness of the formed body is at least 3 HBW 2.5/2, more preferably at least 4 HBW 2.5/2.
  • the Brinell hardness of the formed body is at least 5 HBW 2.5/2 or at least 7 HBW 2.5/2.
  • the size of the samples for Brinell hardness measurement can be 20 ⁇ 20 ⁇ 10 mm.
  • the total content of the water-soluble boron compound(s) in the material composition and the formed body according to the invention can be expressed as content of boric acid in percent by weight, based on the total weight of the formed body.
  • the total content of the water-soluble boron compound(s) of the material composition and the formed body is preferably at least 3% by weight, more preferably at least 5% by weight, more preferably at least 8% by weight, wherein the content of water-soluble boron compound(s) is expressed as content of boric acid in percent by weight, based on the total weight of the material composition or respectively of the formed body.
  • the total content of water-soluble boron compound(s) of the material composition and the formed body is at least 15% by weight, wherein the content of water-soluble boron compound(s) is expressed as content of boric acid in percent by weight, based on the total weight of the material composition or respectively of the formed body.
  • the content of water-soluble boron compound(s) can be expressed as content of boric acid (H 3 BO 3 ) in percent by weight and can be determined by eluting the water-soluble boron compound(s) with water at room temperature and subsequent analysis by ICP-OES (Inductively Coupled Plasma Optical Emission Spectrometry) according to the method described in the following.
  • ICP-OES Inductively Coupled Plasma Optical Emission Spectrometry
  • the sample is used in the as delivered state.
  • Formed and heat-treated formed bodies shall be milled to pass a 300 ⁇ m (50 mesh) sieve. During milling, the energy input shall be as low as possible to minimize sample oxidation and it has to be paid attention that no contamination with boron occurs from the milling tools.
  • boric acid H 3 BO 3
  • ammonium borates other water-soluble boron compound(s) such as ammonium borates.
  • the content of the further inorganic compound in the material composition and the formed body according to the invention preferably is at least 0.5% by weight, more preferably at least 4% by weight, more preferably at least 15% by weight, more preferably at least 25% by weight, based on the total weight of the material composition or respectively of the formed body. In some embodiments, the content of the further inorganic compound in the formed body is at least 50% by weight, based on the total weight of the material composition or respectively of the formed body.
  • boehmite is preferably used as the further inorganic compound in the material composition and the formed body. If boehmite is used as further inorganic compound, the molar ratio of boehmite and water-soluble boron compound(s) in the material composition and in the formed body is preferably from 0.3:1 to 6:1, more preferably from 1:1 to 2:1. The molar ratio of boehmite and water-soluble boron compound(s) can be expressed as molar ratio of AlO(OH):H 3 BO 3 .
  • the heat-treated formed body according to the invention comprises hexagonal boron nitride and the reaction product between a water-soluble boron compound and a further inorganic compound.
  • the further inorganic compound is able to split off a gaseous phase at a heat treatment at a temperature of at most 1000° C., and the further inorganic compound is able to form with the water-soluble boron compound(s) a water-insoluble boron compound at a heat treatment at a temperature of 200-1000° C.
  • the reaction product between the water-soluble boron compound(s) and the further inorganic compound is formed if the reaction partners are heat-treated at a temperature of 200-1000° C.
  • the heat-treated formed body is obtained by heat-treating the formed body according to the invention at a temperature of 200-1000° C.
  • the heat-treated formed body comprises hexagonal boron nitride and the reaction product between the water-soluble boron compound and the further inorganic compound as described above.
  • the reaction product between the water-soluble boron compound and the further inorganic compound is a water-insoluble boron compound.
  • the gaseous phase is water and/or carbon dioxide. More preferably, the gaseous phase is water.
  • the water-soluble boron compound and the further inorganic compound can be selected as described above for the material composition and the formed body.
  • the water-insoluble boron compound that is formed as reaction product between the water-soluble boron compound(s) and the further inorganic compound is preferably a water-insoluble borate, preferably a borate of aluminum, calcium or magnesium. If boehmite (AlO(OH)) is used as further inorganic compound, then the reaction product between the water-soluble boron compound(s) and the further inorganic compound is the aluminum borate Al 4 B 2 O 9 . Preferably, the reaction product between the water-soluble boron compound(s) and the further inorganic compound is the aluminum borate Al 4 B 2 O 9 .
  • the aluminum borate Al 4 B 2 O 9 is a water-insoluble boron compound.
  • the aluminium borate Al 4 B 2 O 9 is formed according to the following reaction: 4AlO(OH)+2H 3 BO 3 ⁇ Al 4 B 2 O 9 +5H 2 O (I)
  • aluminium borate Al 4 B 2 O 9 is also formed when aluminium hydroxide (Al(OH) 3 ) is used as the further inorganic compound. If a hydroxide or oxyhydroxide of calcium or magnesium is used as the further inorganic compound, then a calcium borate or a magnesium borate is formed as the water-insoluble boron compound.
  • the Al 4 B 2 O 9 phase can be detected by X-ray diffraction analysis after heat treatment of at least 730° C. in air and 2 hours dwelling time.
  • the heat-treated formed body may also contain a water-soluble boron compound.
  • the total content of the water-soluble boron compound(s) in the heat-treated formed body can be expressed as content of boric acid in percent by weight and can be determined by eluting the water-soluble boron compound(s) with water at room temperature and subsequent analysis by ICP-OES according to the method described above for the formed body.
  • the total content of water-soluble boron compound(s) in the heat-treated formed body is preferably at most 15% by weight, expressed as content of boric acid in percent by weight, based on the total weight of the heat-treated formed body. More preferably, the total content of water-soluble boron compound(s) of the heat-treated formed body is at most 10%, even more preferably at most 8%, most preferably at most 5% by weight, expressed as content of boric acid in percent by weight, based on the total weight of the heat-treated formed body.
  • the water-soluble boron compound(s) in the heat-treated formed body can be decomposition products of boric acid and ammonium borates, such as metaboric acid or boric oxide, or boric acid that may have been formed by hydrolysis of boric oxide.
  • the heat-treated formed body may also contain aluminium oxide (Al 2 O 3 ).
  • the aluminium oxide can be formed from boehmite by the heat-treatment of the formed body and can be present in the heat-treated formed body as an over-stoichiometric quantity that may not have reacted with the water-soluble boron compound(s).
  • the content of aluminium oxide is preferably at most 35% by weight, more preferably at most 20% by weight, more preferably at most 5% by weight, most preferably at most 2% by weight, based on the total content of the heat-treated formed body. If the hydroxides or oxyhydroxides of other metals than aluminum are used as further inorganic compound, then metal oxides of these other metals may also be present in the heat-treated formed body, for example calcium oxide or magnesium oxide.
  • the heat-treated formed body may contain other inorganic non-metallic components.
  • the heat-treated formed body may contain mineral or ceramic powders, such as oxide ceramics, for example silicon oxide (SiO 2 ) and magnesium oxide (MgO), carbide ceramics, for example silicon carbide (SiC), and nitride ceramics, for example silicon nitride (Si 3 N 4 ) and aluminum nitride (AlN), and mixtures thereof.
  • the mean particle size (d 50 ) of the mineral or ceramic powders is from 0.5 to 10 ⁇ m.
  • the reaction product between the water-soluble boron compound(s) and the further inorganic compound is formed if the reaction partners are heat-treated at a temperature of 200-1000° C.
  • the heat-treatment is performed at a temperature of 300-900° C., more preferably of 400-850° C., most preferably of 600-850° C.
  • the heat-treated formed body has a boron nitride content of at least 15% by weight, preferably at least 40% by weight, more preferably at least 60% by weight, based on the total weight of the heat-treated formed body.
  • the heat-treated formed body preferably has a boron nitride content of at most 95%, more preferably of at most 90%, more preferably of at most 85% by weight, based on the total weight of the heat-treated formed body.
  • the heat-treated formed body has a boron nitride content of at least 60% and at most 85% by weight, based on the total weight of the heat-treated formed body.
  • the density of the heat-treated formed body is at least 1.3 g/cm 3 , preferably at least 1.4 g/cm 3 . In some embodiments, the density of the heat-treated formed body is at least 1.5 or 1.6 g/cm 3 .
  • the relative density of the heat-treated formed body is preferably at least 50% of the theoretical density of the heat-treated formed body. More preferably, the relative density of the heat-treated formed body is at least 55%, more preferably at least 60% of the theoretical density of the heat-treated formed body. Typically, the relative density of the heat-treated formed body can be 99% or less, or 95% or less, or 90% or less, of the theoretical density of the heat-treated formed body.
  • the theoretical density of hexagonal boron nitride heat-treated formed body is calculated by the powder density of the pure hexagonal boron nitride powder which is 2.25 g/cm 3 and the theoretical densities of the other components and their respective fractions in the composition of the formed body.
  • the heat-treated formed body is mechanically stable and has good mechanical properties.
  • the compressive strength of the heat-treated formed body preferably is at least 4 N/mm 2 . More preferably, the compressive strength of the heat-treated formed body is at least 10 N/mm 2 , more preferably at least 15 N/mm 2 . In some embodiments, the compressive strength of the heat-treated formed body is at least 20 N/mm 2 or at least 25 N/mm 2 .
  • the compressive strength is measured as maximum force before rupture of a test sample of the heat-treated formed body, divided by the area of the test sample on which the load is applied. The compressive strength is measured in the pressure test as described above for the formed body.
  • the hardness of the heat-treated formed body can be measured by the Brinell method according to DIN EN ISO 6506-1 (2013).
  • the Brinell hardness of the heat-treated formed body is at least 2 HBW 2.5/2.
  • the designation “HBW 2.5/2” means that the measurement has been carried out with a Brinell sphere of diameter 2.5 mm and with a load of 2 kp.
  • the Brinell hardness of the heat-treated formed body is at least 3 HBW 2.5/2, more preferably at least 5 HBW 2.5/2, most preferably at least 8 HBW 2.5/2.
  • the size of the samples for Brinell hardness measurement can be 20 ⁇ 20 ⁇ 10 mm.
  • the water-soluble boron compounds boric acid (H 3 BO 3 ) and ammonium borates are solid.
  • the ammonium borates decompose at about 100° C. to boric acid and ammonia.
  • boric acid begins to release hydrated water.
  • boric acid can be dissolved and can be transported in water vapour, as a water vapour volatile compound.
  • the boric acid melts and starts to transform to metaboric acid (HBO 2 ).
  • Metaboric acid itself is solid at about 200° C. By further increasing the temperature, metaboric acid melts at about 236° C. and starts to dehydrate above 300° C. and finally transforms to boric oxide at about 400° C.
  • the transformation temperatures are dependent on the heating rate and on the partial pressure of water. Having boehmite, preferably hydrated boehmite, and boric acid in the material composition, further to the release of hydrated water from boric acid, additional water is released from the hydrated boehmite. Dehydration of the physically bound water of boehmite happens at about 100° C. and chemical decomposition of boehmite at about 400-450° C. The released water of boehmite, or of other hydroxides or oxyhydroxides, increases the mobility of boric acid and metaboric acid in formed bodies.
  • boehmite or other hydroxides or oxyhydroxides, preferably hydrated boehmite, and boric acid in the material composition, the water content and the partial pressure of water at elevated temperatures is increased, and thereby the reverse reaction of decomposition products of boric acid such as metaboric acid is supported.
  • This reverse reaction increases the temperature interval with highly mobile modifications of boron compounds and extends the time for highly mobile boron compounds such as metaboric acid and boric acid to diffuse to the outer areas of the formed body, for example if used as sealing component.
  • the highly mobile boron compounds are either liquid or in the gaseous phase, dependent on the temperature.
  • the highly mobile boron compounds at the outer areas of the sealing component fill gaps between the sealing component and the surrounding construction of the sealing component, and this leads to enhanced sealing properties and gas tightness.
  • the highly mobile boron compounds react with boehmite to aluminum borate (Al 4 B 2 O 9 ) at temperatures above 600° C.
  • the water-insoluble boron compound Al 4 B 2 O 9 is also formed at the interfaces of the sealing component with the surrounding construction.
  • boric acid and metaboric acid can be further increased if water releasing compounds like hydroxides and oxyhydroxides are used in combination with carbon dioxide releasing compounds like carbonates of alkali metals or alkaline earth metals as further inorganic compound in the material composition disclosed herein.
  • the leakage rate of the formed body and of the heat-treated formed body is typically below 1000 ⁇ l/min, preferably below 500 ⁇ l/min, more preferably below 100 ⁇ l/min, most preferably below 40 ⁇ l/min.
  • the gas tightness of the formed body and of the heat-treated formed body is unexpected to one skilled in the art.
  • Conventional pressed or isostatically pressed boron nitride powder typically has a porosity of 30-50%.
  • the porosity of the formed body disclosed herein is typically about 10-30%, even subsequent to isostatically pressing at 4000 bar.
  • the porosity increases during heat treatment at temperatures above 400° C. by decomposition of boehmite as decomposition products of boehmite and particularly of hydrated boehmite typically have a porosity of 30-50%.
  • the decomposition products of other further inorganic compounds used herein also have a high porosity.
  • the formed body and the heat-treated formed body can be used, for example as sealing component, at room temperature (23° C.), at room temperature to 100° C., at temperatures of 100-400° C., of 400-700° C., of 700-800° C., of 600-850° C. and of 800-1000° C.
  • the formed body according to the invention is obtainable by a process comprising the steps
  • step (b) generating at least one water soluble boron compound by mechanically pre-treating the powder of step (a) and/or by adding at least one water-soluble boron compound to the powder provided in step (a), thereby obtaining a powder comprising hexagonal boron nitride powder and at least one water-soluble boron compound;
  • step (c) adding to the powder of step (b) a further inorganic compound, wherein the further inorganic compound is able to split off a gaseous phase at a heat treatment at a temperature of at most 1000° C., and wherein the further inorganic compound is able to form with the water-soluble boron compound(s) a water-insoluble boron compound at a heat treatment at a temperature of 200-1000° C.;
  • step (d) mixing the powder of step (b) and the further inorganic compound added in step (c), thereby obtaining a material composition comprising hexagonal boron nitride;
  • the heat-treated formed body according to the invention is obtainable by a process comprising the steps
  • step (b) generating at least one water-soluble boron compound by mechanically pre-treating the powder of step (a) and/or by adding at least one water-soluble boron compound to the powder provided in step (a), thereby obtaining a powder comprising hexagonal boron nitride powder and at least one water-soluble boron compound;
  • step (c) adding to the powder of step (b) a further inorganic compound, wherein the further inorganic compound is able to split off a gaseous phase at a heat treatment at a temperature of at most 1000° C., and wherein the further inorganic compound is able to form with the water-soluble boron compound(s) a water-insoluble boron compound at a heat treatment at a temperature of 200-1000° C.;
  • step (d) mixing the powder of step (b) and the further inorganic compound added in step (c), thereby obtaining a material composition comprising hexagonal boron nitride;
  • a material composition comprising hexagonal boron nitride powder is formed into a shape.
  • the material composition which is suitable for making the formed body is obtainable by a process comprising the steps
  • step (b) generating at least one water-soluble boron compound by mechanically pre-treating the powder of step (a) and/or by adding at least one water-soluble boron compound to the powder provided in step (a), thereby obtaining a powder comprising hexagonal boron nitride powder and at least one water-soluble boron compound;
  • step (c) adding to the powder of step (b) a further inorganic compound, wherein the further inorganic compound is able to split off a gaseous phase at a heat treatment at a temperature of at most 1000° C., and wherein the further inorganic compound is able to form with the water-soluble boron compound(s) a water-insoluble boron compound at a heat treatment at a temperature of 200-1000° C.;
  • step (d) mixing the powder of step (b) and the further inorganic compound added in step (c), thereby obtaining a material composition comprising hexagonal boron nitride;
  • the granulation step (e) is an optional step. Therefore, the material composition comprising hexagonal boron nitride powder which is suitable for making the formed body according to the invention can be a material composition for which a granulation step (e) has been carried out, but it is also possible to use the material composition obtained by steps (a) to (d) to make the formed body.
  • hexagonal boron nitride powders may be used, preferably with a specific surface area (BET) of 1 to 15 m 2 /g. Also powders with a higher or lower specific surface area may be used.
  • the mean particle size (d 50 ) of the hexagonal boron nitride powder may be from 0.2 to 50 ⁇ m, preferably from 0.5 to 25 ⁇ m, more preferably from 1 to 20 ⁇ m, even more preferably from 1 to 15 ⁇ m.
  • the mean particle size (d 50 ) can be measured by laser diffraction (wet measurement, Master Sizer 2000, Malvern).
  • the total oxygen content of the hexagonal boron nitride powder is at most 10%, more preferably at most 8% and even more preferably at most 6%. In some embodiments, the total oxygen content of the hexagonal boron nitride powder is at most 5%, more preferably at most 4%.
  • the oxygen content of the hexagonal boron nitride powder is partially related to oxygen bonded in the lattice of the boron nitride.
  • An additional source of oxygen is related to water-soluble boron compounds such as boric acid (H 3 BO 3 ) and ammonium borates.
  • the graphitization index of the starting powder to be provided in step (a) preferably is at least 1, more preferably at least 1.5, more preferably at least 2.
  • the graphitization index is measured by powder X-ray diffraction.
  • the starting powder to be provided in step (a) comprises less than 50% by volume, preferably less than 20% by volume, based on the total volume of the starting powder, of a powder that has been obtained by machining or milling of hot-pressed hexagonal boron nitride sintered parts. More preferably, the starting powder to be provided in step (a) does not comprise a powder that has been obtained by machining or milling of hot-pressed hexagonal boron nitride sintered parts. Therefore, the formed body comprises less than 50% by volume, preferably less than 20% by volume, more preferably 0% by volume, based on the total volume of the formed body, of a powder that has been obtained by machining or milling of hot-pressed hexagonal boron nitride sintered parts.
  • the graphitization index of powders that have been obtained by machining or milling of hot-pressed hexagonal boron nitride sintered parts is typically below 2.
  • Hexagonal boron nitride powders that have been obtained from hot-pressed hexagonal boron nitride sintered parts by machining or by milling often contain impurities and hard agglomerates and are not well-defined in particle size and particle size distribution.
  • hot-pressing of hexagonal boron nitride is a cost-intensive process. In the process for making the formed body disclosed herein, it is not necessary to use a powder which has been produced by the cost-intensive process of hot-pressing and milling or machining.
  • the hexagonal boron nitride powder provided in step (a) is subjected to a pre-treatment in step (b).
  • the pre-treatment comprises an energy input into the material, preferably a mechanical energy input.
  • the input of mechanical energy may be done by vibrations or impacts or collisions.
  • the mechanical pre-treatment may be a dry milling process, for example a dry ball milling process.
  • any dry mill can be used, for example a ball mill, a dry agitated ball mill or a vibration mill.
  • the atmosphere of the ball-milling process may be oxygen-containing or oxygen-free.
  • the dry-milling is performed typically for at least 30 minutes up to 5 hours.
  • the mean particle size (d 50 ) of the hexagonal boron nitride powder may decrease, but it is also possible that the mean particle size remains unchanged or changes only slightly.
  • the mean particle size is measured by laser diffraction (wet measurement, Mastersizer 2000, Malvern).
  • the mean particle size (d 50 ) of the pre-treated hexagonal boron nitride powder may be from 0.2 to 50 ⁇ m, or from 0.5 to 25 ⁇ m, or from 1 to 20 ⁇ m, or from 3 to 20 ⁇ m.
  • Powder X-ray diffraction measurements of the pre-treated powder show the diffraction pattern of the hexagonal boron nitride crystal structure.
  • the specific surface area (BET) of the hexagonal boron powder undergoes an increase by the pre-treatment of step (b), by a factor of at least 1.5 times of the specific surface area (BET) of the starting material provided in step (a).
  • the factor of the increase of the specific surface area is at least 2, more preferably at least 5, most preferably at least 10.
  • the specific surface area (BET) of the pre-treated powder obtained in step (b) is at least 12 m 2 /g, preferably at least 15 m 2 /g, more preferably at least 20 m 2 /g, more preferably at least 25 m 2 /g, more preferably at least 30 m 2 /g.
  • the specific surface area (BET) has values of at least 50 m 2 /g, or of at least 60 m 2 /g, or of at least 70 m 2 /g. In other embodiments, the specific surface area (BET) has values of at least 75 m 2 /g, or of at least 80 m 2 /g, or of at least 90 m 2 /g, or of at least 100 m 2 /g, or of at least 110 m 2 /g, or of at least 120 m 2 /g.
  • the pre-treated powder obtained by the mechanical pre-treatment of step (b) comprises at least one water-soluble boron compound.
  • the water-soluble boron compound(s) are mainly oxygen containing boron compounds such as boric acid and ammonium borates.
  • Examples of ammonium borates present in the pre-treated powder are ammonium pentaborate (NH 4 )B 5 O 8 or the hydrate thereof. Boric acid (H 3 BO 3 ) and the ammonium pentaborate (NH 4 )B 5 O 8 can be detected by X-ray diffraction measurements.
  • the pre-treated powder does not contain boron oxide (B 2 O 3 ) as boron oxide is preferably not added to the pre-treated powder, and it is not produced by the pre-treatment step. This is confirmed by X-ray measurements that do not show the diffraction lines of boron oxide.
  • the content of water-soluble boron compound(s) is increasing during the pre-treatment of step (b).
  • the increase of the content of water-soluble boron compound(s) by the pre-treatment of step (b) can be as high as at least 1, 2, 5, 6 or 10% by weight, the content of water-soluble boron compound(s) being expressed as content of boric acid in percent by weight, based on the total weight of the pre-treated powder.
  • the mechanical pre-treatment of step (b) is an optional treatment. Preferably, a mechanical pre-treatment is performed. If the hexagonal boron nitride powder provided in step (a) is not subjected to a pre-treatment in step (b), at least one water-soluble boron compound is added to the hexagonal boron nitride powder, for example boric acid (H 3 BO 3 ) or ammonium borates such as ammonium pentaborate (NH 4 )B 5 O 8 or the hydrate thereof. These water-soluble boron compounds can be added in powder form or as aqueous solution. It is also possible to add the water-soluble boron compounds as a mixture with the further inorganic compound that is added in step (c).
  • a water-soluble boron compound for example boric acid (H 3 BO 3 ) or ammonium borates such as ammonium pentaborate (NH 4 )B 5 O 8 or the hydrate thereof.
  • step (b) If the hexagonal boron nitride powder provided in step (a) is subjected to a pre-treatment in step (b), it is also possible to add at least one water-soluble boron compound to the pre-treated powder, before or after the pre-treatment.
  • the graphitization index of the pre-treated powder of step (b) preferably is at least 1, more preferably at least 1.5, more preferably at least 2, more preferably at least 2.5.
  • the graphitization index can be increased by the pre-treatment, and the graphitization index of the pre-treated powder can have a value which is 0.2 plus the value of the starting powder or 0.5 plus the value of the starting powder or 1.0 plus the value of the starting powder.
  • the graphitization index is measured by powder X-ray diffraction.
  • step (b) By the pre-treatment of step (b), small particles of a particle size far below 1 ⁇ m are formed.
  • the formation of fines by the pre-treatment can be observed by scanning electron microscopy (SEM).
  • SEM scanning electron microscopy
  • the fines are sub-micron to nano-scale in size. Many of the fine particles have a particle size of 200 nm or less.
  • the total content of water-soluble boron compound(s) of the hexagonal boron nitride powder after the pre-treatment and of the powder obtained in step (b) is preferably at least 1% by weight, more preferably at least 2% by weight, more preferably at least 6% by weight, more preferably at least 10% by weight, more preferably at least 15% by weight, expressed as content of boric acid in percent by weight, based on the total weight of the hexagonal boron nitride powder after the pre-treatment or respectively of the powder obtained in step (b).
  • the total content of water-soluble boron compound(s) is determined by eluting the water-soluble boron compound(s) with water at room temperature and subsequent analysis by ICP-OES according to the method described above for the formed body.
  • step (c) a further inorganic compound is added that is able to split off a gaseous phase at a heat treatment at a temperature of at most 1000° C.; and the further inorganic compound is able to form with the water-soluble boron compound(s) a water-insoluble boron compound at a heat treatment at a temperature of 200-1000° C.
  • the gaseous phase is water and/or carbon dioxide. More preferably, the gaseous phase is water.
  • the further inorganic compound can be selected as described above for the material composition and the formed body.
  • the further inorganic compound can be added in powder form. It is also possible to add the further inorganic compound in the form of a sol or in the form of a slurry.
  • the sol of the further inorganic compound can be a boehmite sol with mean particle sizes of the dispersed particles below 100 nm and can be obtained by dispersing commercially available spray-dried products such as Disperal and Disperal P2 from Sasol Germany GmbH. It is also possible to add these commercially available spray-dried products in powder form to the powder obtained from step (b).
  • the slurry of the further inorganic compound can be obtained by dispersing the further inorganic compound, for example boehmite (AlO(OH)), magnesium hydroxide (Mg(OH) 2 ) or calcium hydroxide (Ca(OH) 2 ), in a solvent, preferably in water. If the further inorganic compound has been added in powder form, then the resulting material composition obtained in step (d) is in powder form. If the further inorganic compound has been added in the form of a sol or a slurry, then the resulting material composition obtained in step (d) is in the form of a wet powder, a slurry or in the form of a paste.
  • the further inorganic compound for example boehmite (AlO(OH)
  • Mg(OH) 2 magnesium hydroxide
  • Ca(OH) 2 calcium hydroxide
  • the molar ratio of boehmite and water-soluble boron compound(s) is preferably from 0.3:1 to 6:1, more preferably from 1:1 to 2:1.
  • the molar ratio of boehmite and water-soluble boron compound(s) can be expressed as molar ratio of AlO(OH):H 3 BO 3 .
  • the amount of further inorganic compound that is added in step (c) can be chosen corresponding to a selected molar ratio of AlO(OH):H 3 BO 3 .
  • the content of water-soluble boron compound(s), expressed as content of H 3 BO 3 can be taken from the analyzed content of water-soluble boron compound(s) of the powder obtained in step (b).
  • the molar ratio of Al(OH) 3 and water-soluble boron compound(s) can be expressed as molar ratio of Al(OH) 3 :H 3 BO 3 and is preferably from 0.3:1 to 6:1, more preferably from 1:1 to 2:1.
  • the material composition may contain other inorganic non-metallic components, for example mineral or ceramic powders, such as oxide ceramics, for example aluminum oxide (Al 2 O 3 ), silicon oxide (SiO 2 ), and magnesium oxide (MgO), carbide ceramics, for example silicon carbide (SiC), and nitride ceramics, for example silicon nitride (Si 3 N 4 ) and aluminum nitride (AlN), and mixtures thereof, with a typical mean particle size (d 50 ) of from 0.5 to 10 ⁇ m.
  • the additional inorganic components can be added before and subsequent to the pre-treatment step, or in step (c).
  • step (c) After adding in step (c) the further inorganic compound to the powder of step (b) some additional water may be added for granulation in step (e), if the further inorganic compound has been added in powder form.
  • step (d) After adding the further inorganic compound in step (c), the powder of step (b) and the further inorganic compound added in step (c) are mixed, thereby obtaining in step (d) a material composition comprising hexagonal boron nitride.
  • the mixing can be done for example in a blender or an Eirich mixer, if the material composition is in powder form, or in a suitable mixing aggregate for mixing the slurry or the paste, if the material composition is in the form of a slurry or a paste.
  • a paddle mixer or a dissolver can be used for mixing the slurry.
  • the content of the further inorganic compound in the material composition obtained in step (d) preferably is at least 0.5% by weight, more preferably at least 4% by weight, more preferably at least 15% by weight, more preferably at least 25% by weight, based on the total weight of the material composition. In some embodiments, the content of the further inorganic compound in the material composition is at least 50% by weight, based on the total weight of the material composition.
  • the obtained material composition may be pre-dried before granulation, for example if the granulation step is carried out as a dry granulation step.
  • step (d) is further processed by optionally granulating the material composition in step (e).
  • a granulation step (e) is carried out after the mixing step (d).
  • the granulation can be carried out by various granulation methods, a dry granulation process is possible but also granulation processes known in the art such as spray-drying or fluidized-bed drying.
  • a certain amount of water can be added, preferably in an amount of up to 15 wt.-%, more preferably up to 12 wt.-%, more preferably up to 8 wt.-%, more preferably up to 7 wt.-%, based on the total weight of the material composition. Most preferably, the amount of water added is from 0.5 to 7 wt.-%, based on the total weight of the material composition. If the water content of the mixture exceeds 20 wt.-%, a pre-drying step has to be performed below 80° C.
  • a possible method for dry granulation is compacting the evenly mixed material with a roller compactor at least one time.
  • the material composition of step (d) is fed between two counter rotating rolls with a typical gap width of 400 to 1600 ⁇ m.
  • the material composition of step (d) is fed continuously between the two counter rotating rolls.
  • the material is crushed and screened in order to obtain the material composition of step (e). This can be done by processing the material through screen breakers.
  • the sieve width of the screen breakers may be for example from 0.5 to 5 mm.
  • the material can be forced through a sieve, for example with a sieve width of 200 ⁇ m, preferably followed by a sieving step to remove the fines, for example the fines below 50 ⁇ m. The fines can be recycled to the granulating step.
  • a slurry with a solids content of about 15-35 wt.-% can be prepared.
  • the slurry can be prepared by dispersing the powder obtained in step (b) in a boehmite sol by stirring.
  • Spray-drying can be carried out in air at a temperature of 200-250° C. No additional organic binders are needed.
  • the spray-dried agglomerates can be sieved to remove the coarse particles, for example with a sieve having a sieve width of 315 ⁇ m, giving a free-flowing well compactable granulated powder.
  • the fines are separated by a cyclone and can be recycled for spray-drying.
  • step (e) The granulated material composition thus obtained in step (e) can be used for forming it into a shape in order to obtain the formed body of step (f). If no granulation step (e) is carried out, the material composition obtained in step (d) can be directly used for the forming step (0.
  • the bulk density of the granulated material composition obtained in step (e) preferably is at least 0.5 g/cm 3 , more preferably at least 0.6 g/cm 3 . In some embodiments, the bulk density is at least 0.7 g/cm 3 , or at least 0.8 g/cm 3 .
  • the moisture content of the granulated material composition obtained in step (e) preferably is 15% by weight or less, more preferably 12% by weight or less, more preferably 8% by weight or less, more preferably 7% by weight or less, based on the total weight of the granulated material composition.
  • the moisture content of the granulated material composition is at least 0.2% by weight, based on the total weight of the material composition.
  • the moisture content of the granulated material composition is from 0.2% to 7% by weight, based on the total weight of the granulated material composition.
  • the moisture content can be determined with a moisture balance, after keeping a sample at a temperature of 80° C. for 24 hours.
  • the granulate size of the material composition obtained in step (e) is preferably below 1000 ⁇ m, more preferably below 500 ⁇ m. In some embodiments, the granulate size is 200 ⁇ m and less. In some embodiments, the granulate size may be from 50 to 200 ⁇ m.
  • the granulate size from 50 to 200 ⁇ m means a granulate powder that has been obtained by sieving, i.e. by removing the fines by sieving with a sieve width of 50 ⁇ m and by removing the coarse granulates by sieving with a sieve width of 200 ⁇ m. In spray-dried powders, the granulate size is below 350 ⁇ m.
  • the mean particle size d 50 of the spray-dried granulates is typically from 50 to 70 ⁇ m. The mean particle size d 50 is measured by laser diffraction (dry measurement, Mastersizer 2000, Malvern).
  • step (f) the material composition obtained in step (d) or (e) is formed into a shape, thereby obtaining a formed body.
  • the material composition obtained in step (e) is formed into a shape.
  • the forming is performed preferably by a pressing step, preferably by dry pressing, for example cold-isostatic pressing, typically with pressures of 1000 bar up to 4000 bar, or uniaxial pressing, typically with pressures of 500 bar up to 1500 bar.
  • the forming can be carried out at temperatures from 10 to 40° C., preferably at room temperature (23° C.).
  • organic pressing additives for example polyvinyl alcohol and polyethylene glycol
  • the material composition obtained in step (d) or (e), the formed body obtained in step (f) and the heat-treated body obtained in step (g) preferably do not contain pressing additives, and preferably are free of organic additives.
  • the material composition, the formed body and the heat-treated formed body according to the invention preferably do not contain pressing additives, and preferably are free of organic additives.
  • the forming process of step (f) is carried out with a low compaction factor.
  • the compaction factor is defined by the ratio of the volume of the granulated material composition before forming to the volume of the formed body after forming.
  • the compaction factor can also be calculated by the ratio of the bulk density of the granulated material composition before forming to the geometrical density of the formed body after forming.
  • the low compaction factor observed in step (f) is unusual for boron nitride powders.
  • the compaction factor for commercially available boron nitride powders typically is more than 5 or more than 10, for formed bodies with a density of 70% to 80% of theoretical density.
  • the compaction factor for the material composition of step (e) is typically at most 5, preferably at most 3, more preferably at most 2.7.
  • the compaction factor can be determined with a forming pressure of 1200 bar or 17405 psi, for formed bodies with a density of 70% to 85% of theoretical density.
  • a low compaction factor for the powder used for the forming step is useful because this results in shorter cycle times for pressing, and the venting of the pressing mold is improved.
  • the pressing can be performed with commercially available pressing equipment. Small-volume components can be produced with high-speed presses with an output rate of more than 100 pieces per minute.
  • the formed hexagonal boron nitride body can be mechanically machined. It is also possible to have a near net shape forming process, i.e. the formed body can be obtained directly by the forming step without the need of further mechanically machining.
  • the formed body can be heat treated in step (g).
  • the heat treatment can be conducted in inert atmosphere.
  • the heat treatment can also be conducted at atmospheric pressure and in air.
  • the heat treatment can be carried out at a temperature of 200° C.-1000° C., preferably of 300-900° C., more preferably of 400-850° C., most preferably of 600-850° C.
  • the heat treatment is conducted at atmospheric pressure and in air at a temperature of 200-850° C., preferably of 400-850° C., more preferably of 600-850° C.
  • the dwell time can be 1 hour or 2 hours or more.
  • the heat-treated formed body will remain mechanically stable while increasing the temperature up to at most 1000° C. At temperatures beyond 1035° C., Al 4 B 2 O 9 will decompose into B 2 O 3 and Al 18 O 4 O 33 according to the Al 2 O 3 —B 2 O 3 phase diagram.
  • the water-soluble boron compounds like boric acid and ammonium borates will split off water vapor, and the further inorganic compound will split off a gaseous phase such as carbon dioxide or water vapor, and a water-insoluble boron compound is formed.
  • boehmite is used as further inorganic compound, boehmite will split off water vapor while raising the temperature, and the aluminum borate Al 4 B 2 O 9 is formed.
  • the dehydration and the separation of carbon dioxide causes weight loss and leads to increasing porosity, but no shrinkage will occur.
  • the weight loss of the formed body is in the range of 1 to 30 wt.-%. At a temperature of 600° C. the process of dewatering is completed.
  • the hardness and compressive strength of the formed body does not change significantly up to 1000° C. in inert atmosphere.
  • the heat treatment can be performed in air.
  • the remnant volume expansion of the formed body is more than 0%, preferably at least 1%. In some embodiments, the remnant volume expansion is at least 3% or at least 5%.
  • the remnant volume expansion can be measured on formed bodies having dimensions of 20 ⁇ 20 ⁇ 10 mm and that have been formed by axial pressing with a pressure of 1250 bar, by measuring the volume before and after heat treatment.
  • hexagonal boron nitride shows the behavior of a remnant expansion when a heat treatment is performed, and the remnant expansion of the formed body is only observed if a sufficient amount of hexagonal boron nitride is present in the formed body.
  • the remnant expansion improves the sealing properties of the formed body used as sealing component, as gaps with the surrounding construction are closed during the heat-treatment of the formed body and even the thermal expansion of the surrounding construction can be compensated.
  • the remnant expansion in combination with the mobility enhancement of boric acid by water release of the further inorganic compound leads to unexpected good sealing properties of the formed body and of the heat-treated formed body. This leads to a force- and form-locking connection of the formed body and of the heat-treated formed body with the surrounding construction when used as sealing component.
  • boron nitride If less than 15% by weight of boron nitride is present in the formed body, the remnant expansion will not be observed. With less than 15% by weight of boron nitride in the material composition or in the formed body, other ceramic materials such as oxide and non-oxide materials would be wetted at a heat treatment by boric acid, i.e. by the highly mobile boron compounds, and shrinkage of the formed body would happen during heat treatment. By the shrinkage of the formed body, existing gaps to the surrounding construction would grow and gas tightness would be strongly reduced.
  • the content of water-soluble boron compound(s) in the heat-treated formed body is lower than the content of water-soluble boron compound(s) in the formed body. This reduction of water-soluble boron compound(s) is due to the fact that Al 4 B 2 O 9 is less soluble than the water-soluble boron compound(s).
  • the content of water-soluble boron compound(s) in the formed body and in the heat-treated formed body is measured by ICP-OES as described above.
  • the factor of reduction f RED of the water-soluble boron compound(s) is more than 1, preferably at least 1.2, more preferably at least 1.5, more preferably at least 2.0.
  • step (e) It is possible to shape the material composition obtained in step (e) directly to a formed body in a sealing construction or in a heating cartridge.
  • the formed body is directly produced in the sealing position or in the heating cartridge, and the forming step (f) is carried out directly in the sealing position or in the heating cartridge.
  • the heat treatment of step (g) at a temperature of 200-1000° C. can be carried out for the formed body together with the sealing construction or the heating cartridge. If the forming step (f) is carried out directly in the sealing position or in the heating cartridge, it is also possible to use the material composition obtained in step (e) for which additionally a heat treatment has been carried out at a temperature of 200-1000° C.
  • the formed hexagonal boron nitride body and the heat-treated formed body can be used as or for producing electrically isolating parts, for example electrically isolating parts for furnace manufacturing or electrically isolating parts for high temperature applications. These electrically isolating parts have the advantage of being free of organic additives.
  • the formed hexagonal boron nitride body can also be used as or for producing sealing components, for example as sealing component for sensors, and as or for producing heating cartridges and automotive components such as engine components or exhaust gas components.
  • the formed body and the heat-treated formed body as a sealing component in contact to additional sealing element(s).
  • additional sealing element(s) can for example be made of pressed parts of boehmite or of the other further inorganic compounds used herein.
  • a tube consisting of stainless steel was prepared, having an inner diameter of 9 mm, an outer diameter of 30 mm, a length of 70 mm. At both ends of the tube are internal screw threads with an inner diameter of >9 mm, and external screw threads for connection of air pressure supply for the leakage test.
  • the boron nitride granulated material composition is formed into a cylindrical formed body with a diameter of 8.9 mm and a height of 9 mm by dry pressing with a force of 7 kN. This formed body is inserted into the tube. Inside the stainless steel tube, on both ends of the formed body, cylindrical stainless steel bodies with a diameter of 8.9 mm are placed for transferring the load. One end of the tube is closed with a screw, using the inside thread, thereby fixing the formed body on this end of the tube. The other end is closed with a screw having a longitudinal bore hole of 6.5 mm, using the inside thread.
  • the formed body in the tube is pressed with a force of 20 kN, using a pin with 6 mm diameter, which was inserted into the 6.5 mm bore hole of the screw in order to transfer force from a hand press to the formed body. While applying the pressure, the screw can be retightened, because the height of the formed body is reduced. After the pin has been removed from the bore hole, the leakage rate can be determined with the sealed tube thus obtained as follows.
  • One external screw thread is used to connect an air pressure supply of 4 bars, applying air pressure on the sealed tube.
  • the air that passes through the formed body is quantitatively received by a flexible hose connected to the external screw thread on the other end of the sealed tube.
  • the amount of the air that passes through is measured with a burette that is headfirst put into a water filled vessel.
  • the burette collects the air from the flexible hose, which leads to a drop of the water-level in the burette.
  • the drop of water level is recorded as a function of time and gives a straight line, showing that the leakage rate is constant.
  • the gradient of the straight line gives the leakage rate of the sealed tube and of the formed body in ⁇ l/min.
  • the sealed tube After measuring the leakage rate of the formed body, the sealed tube is heat treated in a furnace at atmospheric pressure to a temperature of 730° C. with a dwell time of 1 h. After natural cool-down, the leakage rate of the heat-treated formed body is measured with the method described above.
  • the boehmite sol is prepared by peptizing the commercially available product “Disperal” (Sasol Germany GmbH) in water.
  • the peptized particles consist of crystallites of 10 nm which contain aluminum oxyhydrate (AlO(OH)) and physically bound water.
  • Disperal has an aluminum oxide yield of 77% by weight (supplier information).
  • the content of pure, water-free AlO(OH) in Disperal is calculated to be 90.6 wt %.
  • boehmite sol based on Disperal The preparation of boehmite sol based on Disperal is carried out as follows: 1000 ml of deionized water is heated in a beaker to a temperature above 80° C. while stirring. 300 g of Disperal powder is added and homogenized for 10 minutes, keeping the temperature above 80° C. 17 g of nitric acid (65%) is added slowly and the dispersion is stirred for at least another 10 minutes, until the fluid appears transparent.
  • the solids content of the so prepared boehmite sol is 23.1 wt.-% Disperal, the AlO(OH) content is 20.9 wt.-%.
  • the boehmite sol is prepared by peptizing the commercially available product “Disperal P2” (Sasol Germany GmbH) in water.
  • the peptized particles consist of crystallites of 4.5 nm, which contain aluminum oxyhydrate (AlO(OH)) and physically bound water.
  • Disperal P2 has an aluminum oxide yield of 72% by weight (supplier information). The content of pure, water-free AlO(OH) in Disperal is calculated to be 84.7 wt.-%.
  • boehmite sol based on Disperal P2 The preparation of boehmite sol based on Disperal P2 is carried out as follows: 1000 ml of deionized water is heated in a beaker to a temperature above 80° C. while stirring. 90.5 g of Disperal P2 is added and homogenized for 10 minutes, keeping the temperature above 80° C. 22.5 g of nitric acid (65%) is added slowly and the dispersion is stirred for at least another 10 minutes, until the fluid appears transparent.
  • the solids content of the so prepared boehmite sol is 8 wt.-% Disperal P2, the AlO(OH) content is 6.8 wt %.
  • a hexagonal boron nitride powder (3MTM Boron Nitride Powder Grade S1X) with a specific surface area of 11.9 m 2 /g, a mean particle size (d 50 ) of 3 ⁇ m and a content of water-soluble boron compound(s) of 9% by weight (expressed as content of H 3 BO 3 ) is used as starting powder.
  • This powder is pre-treated as described in the following.
  • the starting powder is filled into a vibration mill (Vibratom Typ 125, Siebtechnik GmbH, Malheim, Germany), together with hard metal milling-balls with an average diameter of 10 mm.
  • the container of the vibration mill is filled up to 2 ⁇ 3 with milling balls, and about 20-40 kg of starting powder are added.
  • the vibration mill is run for 45 minutes with the starting powder and the milling balls. After this pre-treatment in the vibration mill the specific surface area rises to 36.8 m 2 /g and the content of water-soluble boron compound(s) (expressed as content of H 3 BO 3 ) of the hexagonal boron nitride powder has increased up to a value of about 17% by weight.
  • the molar ratio of AlO(OH) to the water-soluble compound(s) (expressed as H 3 BO 3 ) is selected to be 2:1, taking the H 3 BO 3 content of the pre-treated powder (17 wt.-%) into account.
  • 17.1 wt.-% of the pre-treated powder are mixed with 82.9 wt.-% of the boehmite sol based on Disperal P2, and then additional 4.1 wt.-% deionized water are added and homogeneously mixed for 10 minutes.
  • the granulation is carried out via spray drying (Niro, Mobile Minor 2000).
  • the generated granules are sieved with a sieve having a sieve width of 315 ⁇ m to remove coarse particles, the medium particle size is 57 ⁇ m (d 50 ).
  • the cyclone fines are recycled and re-used for the granulating step.
  • the so produced granules have a bulk density of 0.77 g/cm 3 (see table 1).
  • the granules are uniaxial dry-pressed into a 20 ⁇ 20 mm mold with a force of 50 kN.
  • the compaction factor is 2.35.
  • the obtained formed body with the dimensions 20 ⁇ 20 ⁇ 10 mm shows a geometrically determined density of 1.81 g/cm 3 . A remaining porosity of 19.8% is present.
  • the formed body has a compressive strength of 28.9 MPa and a hardness of 7.4 HBW 2.5/2 (see table 1) and is good to handle with respect to the stability of edges.
  • the content of water-soluble boron compound(s) of the formed body is 9.3 wt.-%.
  • a heat treatment is performed at 730° C. at atmospheric atmosphere for 1 h in a furnace (Nabertherm GmbH, Germany, HT40/17S) and naturally cooled down, thereby obtaining heat-treated formed bodies.
  • the density of the formed bodies having the dimensions 20 ⁇ 20 ⁇ 10 mm has decreased by the heat-treatment to 1.6 g/cm 3 , and the porosity has increased to 32.2%.
  • the results of compressive strength and Brinell hardness measurements of the heat-treated formed body are shown in table 1.
  • the volume of the formed body has increased by the heat treatment, a remnant expansion in volume of 1.3% is observed.
  • the content of water-soluble boron compound(s) after heat treatment has decreased to 3.9 wt.-%.
  • the formation of Al 4 B 2 O 9 is observed by X-ray diffraction when heated to 730° C. after a dwell time of at least 2 hours.
  • the leakage rate was measured following the procedure described above.
  • the leakage rate of the formed body is 0 ⁇ l/min
  • the leakage rate of the heat-treated formed body is 32 ⁇ l/min.
  • the same pre-treated powder is used as in example 1.
  • the molar ratio of AlO(OH) to water-soluble compound(s) (expressed as H 3 BO 3 ) is selected to be 1:1, taking the H 3 BO 3 content of the pre-treated powder (17 wt.-%) into account.
  • 56 wt.-% of the pre-treated powder are mixed with 44 wt.-% of the boehmite sol based on Disperal in an Eirich mixer for 10 minutes at 1200 rpm.
  • this material composition is put into a drying-furnace at 60° C. until the water content has decreased to 19.1 wt.-%.
  • the water content is determined by the weight loss of a 10 g sample after keeping the sample for 15 minutes at 125° C.
  • the granulation is carried out as a dry granulation with a roller compactor (RC 100 PHARMA from Powtec, Remscheid, Germany).
  • a roller compactor RC 100 PHARMA from Powtec, Remscheid, Germany.
  • the dried material composition is continuously fed between two counter rotating structured rolls.
  • the rolls are structured with 1.2 mm deep axial rounded grooves distributed along the circumference of the rolls with a distance of about 10 mm to each other.
  • the two counter rotating rolls have a gap width of 400-1600 ⁇ m and are pressed with a pressure of about 150 bar against each other while rotating with 10 rpm.
  • the roller-compacted material is processed through screen-breakers with a mesh width of 1000 ⁇ m (80 rpm). These two steps are repeated 5 times.
  • the material is forced through a sieve with a sieve width of 200 ⁇ m.
  • the so produced granulated material composition has a bulk density of 0.65 g/cm 3 (see table 1).
  • the granulated material composition is uniaxially dry-pressed into a 20 ⁇ 20 mm mold with a force of 50 kN.
  • the compaction factor is 2.6.
  • the obtained formed body with the dimensions 20 ⁇ 20 ⁇ 10 mm shows a geometrically determined density of 1.70 g/cm 3 . A remaining porosity of 21.5% is present.
  • the formed body has a compressive strength of 8.8 MPa and a Brinell hardness of 3.4 HBW 2.5/2 (see table 1).
  • the content of water soluble boron compound(s) of the formed body is 16.6 wt.-%.
  • the same pre-treated powder is used as in example 1.
  • the molar ratio of AlO(OH) to water-soluble compound(s) (expressed as H 3 BO 3 ) is selected to be 4:1.
  • 23.8 wt.-% of the pre-treated powder are homogeneously mixed with 76.2 wt.-% of boehmite sol based on Disperal in an Eirich mixer for 10 minutes at 1200 rpm.
  • the material composition is dried at 60° C. until the water content has decreased to 9.3 wt.-%.
  • the material composition is granulated as in example 2.
  • the forming of the granulated powder composition is carried out by dry-pressing as in example 2, and the obtained formed body is subjected to a heat treatment as in example 2.
  • the content of water-soluble boron compound(s) of the formed body is 6.6 wt.-%.
  • the volume of the formed body has increased by the heat treatment, a remnant volume expansion of 3.8% is observed.
  • the content of water-soluble boron compound(s) after heat treatment has decreased to 3.0 wt.-%.
  • Example 4 the same pre-treated powder is used as in example 1.
  • the molar ratio of AlO(OH) to water-soluble compound(s) (expressed as H 3 BO 3 ) is selected to be 6:1.
  • 17.2 wt.-% of the pre-treated powder are homogeneously mixed with 82.8 wt.-% of boehmite sol based on Disperal in an Eirich mixer for 10 minutes at 1200 rpm.
  • the material composition is dried at 60° C. until the water content has decreased to 12.7 wt.-%.
  • the material composition is granulated as in example 2.
  • the forming of the granulated material composition is carried out by dry-pressing as in example 2, and the obtained formed body is subjected to a heat treatment as in example 2.
  • the content of water-soluble boron compound(s) of the formed body is 5.7 wt.-%.
  • the volume of the formed body has increased by the heat treatment, a remnant volume expansion of 1.6% is observed.
  • the content of water-soluble boron compound(s) after heat treatment has decreased to 3.9 wt.-%.
  • the leakage rate was measured following the procedure described above.
  • the leakage rate of the formed body is 3 ⁇ l/min
  • the leakage rate of the heat-treated formed body is 700 ⁇ l/min.
  • a commercially available high purity hexagonal boron nitride powder (3MTM Boron Nitride Powder Grade S1) with a specific surface area of 13 m 2 /g, a mean particle size (d 50 ) of 3 ⁇ m, a graphitization index of 3.7 and a content of water-soluble boron compound(s) of not more than 0.08% by weight is used as starting powder.
  • a pre-treatment of this powder is carried out as described in example 1. By this pre-treatment, the specific surface area reaches 45 m 2 /g and the content of water-soluble boron compound(s) of the hexagonal boron nitride powder has increased up to a value of 6.2% by weight (expressed as H 3 BO 3 ).
  • the content of water-soluble boron compound(s) of the formed body is 18.3 wt.-%.
  • the volume of the formed body has increased by the heat treatment, a remnant volume expansion of 5.6% is observed.
  • the content of water-soluble boron compound(s) after heat treatment has decreased to 12.4 wt.-%.
  • the leakage rate was measured following the procedure described above.
  • the leakage rate of the formed body is 4 ⁇ l/min
  • the leakage rate of the heat-treated formed body is 435 ⁇ l/min.
  • the same pre-treated powder is used as in example 2.
  • 93 wt.-% of the pre-treated powder are homogeneously mixed with 7 wt.-% of deionized water in an Eirich mixer for 10 minutes at 1200 rpm. No boehmite or other further inorganic compound is added.
  • the obtained powder is granulated as in example 2.
  • the forming of the granulated powder is carried out by dry-pressing as in example 2, and the obtained formed body is subjected to a heat treatment as in example 2.
  • the content of water-soluble boron compound(s) of the formed body is 15.5 wt.-%.
  • the volume of the formed body has increased by the heat treatment, a remnant volume expansion of 8.1% is observed.
  • the content of water-soluble boron compound(s) after heat treatment has increased to 17.2 wt.-%.
  • the leakage rate was measured following the procedure described above.
  • the leakage rate of the formed body is 0 ⁇ l/min
  • the leakage rate of the heat-treated formed body is 48 ⁇ l/min.
  • the Brinell hardness of the heat-treated formed body measured with 1 kp is 2.1 HBW 2.5/1, due to the low hardness it was not possible to measure the hardness with 2 kp as for the other examples.
  • the same pre-treated powder is used as in example 5.
  • the molar ratio of AlO(OH) to water-soluble compound(s) (expressed as H 3 BO 3 ) is selected to be 2:1.
  • the material composition is granulated as in example 2.
  • the forming of the granulated material composition is carried out by dry-pressing as in example 2, and the obtained formed body is subjected to a heat treatment as in example 2.
  • the content of water-soluble boron compound(s) of the formed body is 25.1 wt.-%.
  • the volume of the formed body has decreased by the heat treatment, a shrinkage in volume of 1.1% is observed.
  • the content of water-soluble boron compound(s) after heat treatment has decreased to 5.7 wt.-%.
  • the leakage rate was measured following the procedure described above.
  • the leakage rate of the formed body is 0 ⁇ l/min
  • the leakage rate of the heat-treated formed body is higher than 5000 ⁇ l/min, corresponding to the observed shrinkage of the formed body.
  • a material composition comprising hexagonal boron nitride, a water-soluble boron compound and a further inorganic compound, wherein the further inorganic compound is able to split off a gaseous phase at a heat treatment at a temperature of at most 1000° C., and wherein the further inorganic compound is able to form with the water-soluble boron compound(s) a water-insoluble boron compound at a heat treatment at a temperature of 200-1000° C.
  • the material composition according to embodiment 1, wherein the gaseous phase is water and/or carbon dioxide, preferably water.
  • a formed body comprising the material composition of any of embodiments 1 to 6.
  • the heat-treated formed body according to any of embodiments 11 to 13, wherein the heat-treated formed body has a Brinell hardness of at least 3 HBW 2.5/2, preferably at least 5 HBW 2.5/2, most preferably at least 8 HBW 2.5/2, and wherein the Brinell hardness is measured according to DIN EN ISO 6506-1 (2013).
  • the heat-treated formed body according to any of embodiments 11 to 15, wherein the heat-treated formed body has a boron nitride content of at least 15% by weight, preferably at least 40% by weight, more preferably at least 60% by weight, based on the total weight of the heat-treated formed body.
  • step (b) generating at least one water soluble boron compound by mechanically pre-treating the powder of step (a) and/or by adding at least one water-soluble boron compound to the powder provided in step (a), thereby obtaining a powder comprising hexagonal boron nitride powder and at least one water-soluble boron compound;
  • step (c) adding to the powder of step (b) a further inorganic compound, wherein the further inorganic compound is able to split off a gaseous phase at a heat treatment at a temperature of at most 1000° C., and wherein the further inorganic compound is able to form with the water-soluble boron compound(s) a water-insoluble boron compound at a heat treatment at a temperature of 200-1000° C.;
  • step (d) mixing the powder of step (b) and the further inorganic compound added in step (c), thereby obtaining a material composition comprising hexagonal boron nitride;
  • step (b) generating at least one water soluble boron compound by mechanically pre-treating the powder of step (a) and/or by adding at least one water-soluble boron compound to the powder provided in step (a), thereby obtaining a powder comprising hexagonal boron nitride powder and at least one water-soluble boron compound;
  • step (c) adding to the powder of step (b) a further inorganic compound, wherein the further inorganic compound is able to split off a gaseous phase at a heat treatment at a temperature of at most 1000° C., and wherein the further inorganic compound is able to form with the water-soluble boron compound(s) a water-insoluble boron compound at a heat treatment at a temperature of 200-1000° C.;
  • step (d) mixing the powder of step (b) and the further inorganic compound added in step (c), thereby obtaining a material composition comprising hexagonal boron nitride;
  • step (b) is a dry milling process.

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